Calcium plays a vital role in cell function and survival. For example, calcium is a key element in the transduction of signals into and within cells. Cellular responses to growth factors, neurotransmitters, hormones and a variety of other signal molecules are initiated through calcium-dependent processes. Many proteins are activated by binding calcium and in turn affect other proteins in signal cascade mechanisms in cells. The normal basal concentration of free calcium in the cytoplasm of cells is about 50-100 nM whereas the extracellular calcium concentration is typically about 2 mM. Therefore, intracellular calcium levels and fluctuations thereof are tightly regulated by cells.
Calcium regulation by cells is accomplished through a variety of mechanisms, some of which are associated with particular cell types. For example, excitable cells, such as muscle and nerve cells in which calcium signals are essential to functions including contraction and transmission of nerve impulses, contain voltage-gated calcium channels spanning the cell membrane. These channels respond to depolarization of the potential difference across the membrane and can open to permit an influx of calcium from the extracellular medium and a rapid increase in intracellular calcium concentrations.
Nonexcitable cells, e.g., blood cells, fibroblasts and epithelial cells, as well as many excitable cells, contain channels that span intracellular membranes and that can open to permit an influx of calcium into the cytoplasm from calcium-storing organelles, such as the endoplasmic reticulum. One such intracellular ion channel is the inositol 1,4,5-triphosphate (IP3) receptor located in the membrane of the endoplasmic reticulum. The IP3 receptor functions as a ligand-gated ion channel that permits passage of calcium upon binding of IP3 released through hydrolysis of membrane phospholipids by activated phospholipase C (PLC). PLC can be activated through agonist binding to a surface membrane G protein-coupled receptor. Activation of the IP3 receptor results in the release of calcium stored in the endoplasmic reticulum into the cytoplasm. Reduced endoplasmic reticulum calcium concentration resulting from release of calcium therefrom provides a signal for influx of calcium from the extracellular medium into the cell. It appears that this influx of calcium does not rely on voltage-gated plasma membrane channels and does not involve activation of calcium channels by calcium. This calcium influx mechanism has been referred to as capacitative calcium entry (CCE) or store-operated calcium entry (SOCE). The actual factor that directly activates influx of calcium across the plasma membrane in CCE is unknown, as is the identity of the molecule or molecules that provide for mobilization of calcium across the plasma membrane and into the cell.
Because of the vital role that calcium plays in cell function and survival, dysregulation of calcium in cells can have deleterious effects on cell structure and function. Alterations in intracellular calcium homeostasis have been implicated in a variety of diseases.
There is a need, therefore, to elucidate the factors, structures and mechanisms involved in calcium regulation in cells, which may be targets for therapeutic intervention in diseases associated with calcium dysregulation. There is also a need for agents that modulate intracellular calcium and methods of identifying such agents as possible therapeutic compounds for treatment of diseases associated with calcium dysregulation.